In recent years, in response to the increase in demand for optical communication services, in a wavelength division multiplexing (WDM) system, many light sources having many wavelengths are required in order to bind 40 to 80 wavelength signals into around 32-nm band of an optical fiber amplifier. Therefore, there was a problem that many backup light sources should be prepared for use during network failure.
Moreover, in order to increase the flexibility of a network, there is an increasing demand to provide the function of changing a route of respective wavelength signals by remote control. From this respect, it is desirable to have a wavelength-variable light source capable of emitting light having wavelength variable over a wide range covering the band of an optical fiber amplifier.
Hitherto, several wavelength-variable light sources capable of satisfying the demand have been implemented: however, all of them had a complex structure and were difficult to mass-produced and expensive. Therefore, it is desirable to implement a wavelength-variable light source suitable for mass production.
An example of a conventional wavelength-variable optical transmission module is disclosed in Non-Patent Document 1 or 2. The optical transmission module disclosed in the Non-Patent Documents has a structure called an external cavity type.
Another example of the conventional wavelength variable optical transmission module includes “Laser Module” disclosed in Patent Document 1, “Multi-Channel Light Source and Multi-Channel Optical Module Using the Same” disclosed in Patent Document 2, “Optical Module” disclosed in Patent Document 3, and the like.    Patent Document 1:    Japanese Patent Application Laid-Open (JP-A) No. 2003-110190    Patent Document 2: JP-A No. 2005-150724    Patent Document 3: JP-A No. 2005-352230    Non-Patent Document 1:    http://download.intel.com/design/network/ProdBrf/30081902.pdf    Non-Patent Document 2:    http://download.intel.com/design/network/products/optical/prodbrf/30664 501.pdf